CAPEC-197: Exponential Data Expansion
Also known as: Billion Laughs Attack, XML Bomb, XML Entity Expansion (XEE)
An adversary submits data to a target application which contains nested exponential data expansion to produce excessively large output. Many data format languages allow the definition of macro-like structures that can be used to simplify the creation of complex structures. However, this capability can be abused to create excessive demands on a processor's CPU and memory. A small number of nested expansions can result in an exponential growth in demands on memory.
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Overview
CAPEC-197 (Exponential Data Expansion) is a detailed-level attack pattern catalogued by MITRE in the Common Attack Pattern Enumeration and Classification (CAPEC). It describes a recurring method attackers use to exploit software weaknesses.
How the attack works
The phases an attacker typically follows to carry out this attack.
- Step 1Explore
[Survey the target] An adversary determines the input data stream that is being processed by a data parser that supports using subsitituion on the victim's side.
- Use an automated tool to record all instances of URLs to process requests.
- Use a browser to manually explore the website and analyze how the application processes requests.
- Step 2Experiment
[Craft malicious payload] The adversary crafts a malicious message containing nested exponential expansion that completely uses up available server resources. See the "Example Instances" section for details on how to craft this malicious payload.
- Step 3Exploit
[Send the message] Send the malicious crafted message to the target URL.
What the attacker needs
Prerequisites
- This type of attack requires that the target must receive input but either fail to provide an upper limit for entity expansion or provide a limit that is so large that it does not preclude significant resource consumption.
Skills required
- Low skill: Ability to craft nested data expansion messages.
Resources required
- None: No specialized resources are required to execute this type of attack.
Consequences
What a successful CAPEC-197 attack can achieve.
Unreliable Execution, Resource Consumption
Affects: Availability
Denial of Service
How to mitigate it
Defenses that reduce the risk of CAPEC-197.
- Design: Use libraries and templates that minimize unfiltered input. Use methods that limit entity expansion and throw exceptions on attempted entity expansion.
- Implementation: For XML based data - disable altogether the use of inline DTD schemas when parsing XML objects. If a DTD must be used, normalize, filter and use an allowlist and parse with methods and routines that will detect entity expansion from untrusted sources.
Examples
The most common example of this type of attack is the "many laughs" attack (sometimes called the 'billion laughs' attack). For example: ]> &lol9; This is well formed and valid XML according to the DTD. Each entity increases the number entities by a factor of 10. The line of XML containing lol9; expands out exponentially to a message with 10^9 entities. A small message of a few KBs in size can easily be expanded into a few GB of memory in the parser. By including 3 more entities similar to the lol9 entity in the above code to the DTD, the program could expand out over a TB as there will now be 10^12 entities. Depending on the robustness of the target machine, this can lead to resource depletion, application crash, or even the execution of arbitrary code through a buffer overflow.
This example is similar, but uses YAML. This was used to attack Kubernetes [REF-686] a: &a ["lol","lol","lol","lol","lol","lol","lol","lol","lol"] b: &b [*a,*a,*a,*a,*a,*a,*a,*a,*a] c: &c [*b,*b,*b,*b,*b,*b,*b,*b,*b] d: &d [*c,*c,*c,*c,*c,*c,*c,*c,*c] e: &e [*d,*d,*d,*d,*d,*d,*d,*d,*d] f: &f [*e,*e,*e,*e,*e,*e,*e,*e,*e] g: &g [*f,*f,*f,*f,*f,*f,*f,*f,*f] h: &h [*g,*g,*g,*g,*g,*g,*g,*g,*g] i: &i [*h,*h,*h,*h,*h,*h,*h,*h,*h]
Terminology & mappings
Alternate terms
- Billion Laughs Attack
- XML Bomb
- XML Entity Expansion (XEE)
Mapped taxonomies
- WASC: XML Entity Expansion (44)
Frequently asked questions
Common questions about CAPEC-197.
- What is CAPEC-197?
- An adversary submits data to a target application which contains nested exponential data expansion to produce excessively large output. Many data format languages allow the definition of macro-like structures that can be used to simplify the creation of complex structures. However, this capability can be abused to create excessive demands on a processor's CPU and memory. A small number of nested expansions can result in an exponential growth in demands on memory.
- How does a Exponential Data Expansion attack work?
- It typically unfolds over 3 phases. It begins with: [Survey the target] An adversary determines the input data stream that is being processed by a data parser that supports using subsitituion on the victim's side.
- How do you prevent CAPEC-197?
- Design: Use libraries and templates that minimize unfiltered input. Use methods that limit entity expansion and throw exceptions on attempted entity expansion.
- What weaknesses does CAPEC-197 target?
- CAPEC-197 exploits 2 CWE weaknesses, including CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-776 (Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')).
- How severe is CAPEC-197?
- MITRE rates CAPEC-197 as Medium severity with high likelihood of attack.
References
Attack-pattern data is sourced from the MITRE CAPEC catalog (v3.9). Weakness associations link to the corresponding CWE entries on RadicalNotion.AI.
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